solve_bicgstab: use fewer MFs (#3635)

## Summary

This PR cuts the number of MFs used in `solve_bicgstab`, saving on
memory and LocalCopy operations. In particular, the MFs `ph` and `sh`
are removed.

## Additional background

This is a follow up to avoid-use-of-s and other PRs to improve
`solve_bicgstab`. My own testing has shown that this PR gives the same
results as before, but regression testing should be done to verify this
in all cases.

Src/LinearSolvers/MLMG/AMReX_MLCGSolver.H

@@ -90,14 +90,12 @@ MLCGSolverT<MF>::solve_bicgstab (MF& sol, const MF& rhs, RT eps_rel, RT eps_abs)
 
     const int ncomp = sol.nComp();
 
-    MF ph = Lp.make(amrlev, mglev, sol.nGrowVect());
-    MF sh = Lp.make(amrlev, mglev, sol.nGrowVect());
-    ph.setVal(RT(0.0));
-    sh.setVal(RT(0.0));
+    MF p = Lp.make(amrlev, mglev, sol.nGrowVect());
+    MF r = Lp.make(amrlev, mglev, sol.nGrowVect());
+    p.setVal(RT(0.0)); // Make sure all entries are initialized to avoid errors
+    r.setVal(RT(0.0));
 
     MF sorig = Lp.make(amrlev, mglev, nghost);
-    MF p     = Lp.make(amrlev, mglev, nghost);
-    MF r     = Lp.make(amrlev, mglev, nghost);
     MF rh    = Lp.make(amrlev, mglev, nghost);
     MF v     = Lp.make(amrlev, mglev, nghost);
     MF t     = Lp.make(amrlev, mglev, nghost);
@@ -151,8 +149,7 @@ MLCGSolverT<MF>::solve_bicgstab (MF& sol, const MF& rhs, RT eps_rel, RT eps_abs)
             MF::Saxpy(p, -omega, v, 0, 0, ncomp, nghost); // p += -omega*v
             MF::Xpay(p, beta, r, 0, 0, ncomp, nghost); // p = r + beta*p
         }
-        ph.LocalCopy(p,0,0,ncomp,nghost);
-        Lp.apply(amrlev, mglev, v, ph, MLLinOpT<MF>::BCMode::Homogeneous, MLLinOpT<MF>::StateMode::Correction);
+        Lp.apply(amrlev, mglev, v, p, MLLinOpT<MF>::BCMode::Homogeneous, MLLinOpT<MF>::StateMode::Correction);
         Lp.normalize(amrlev, mglev, v);
 
         RT rhTv = dotxy(rh,v);
@@ -164,9 +161,10 @@ MLCGSolverT<MF>::solve_bicgstab (MF& sol, const MF& rhs, RT eps_rel, RT eps_abs)
         {
             ret = 2; break;
         }
-        MF::Saxpy(sol, alpha, ph, 0, 0, ncomp, nghost); // sol += alpha * ph
-        MF::Saxpy(r,  -alpha, v,  0, 0, ncomp, nghost); // r += -alpha * v
+        MF::Saxpy(sol, alpha, p, 0, 0, ncomp, nghost); // sol += alpha * p
+        MF::Saxpy(r,  -alpha, v, 0, 0, ncomp, nghost); // r += -alpha * v
 
+        rnorm = norm_inf(r);
         rnorm = norm_inf(r);
 
         if ( verbose > 2 && ParallelDescriptor::IOProcessor() )
@@ -179,8 +177,7 @@ MLCGSolverT<MF>::solve_bicgstab (MF& sol, const MF& rhs, RT eps_rel, RT eps_abs)
 
         if ( rnorm < eps_rel*rnorm0 || rnorm < eps_abs ) { break; }
 
-        sh.LocalCopy(r,0,0,ncomp,nghost);
-        Lp.apply(amrlev, mglev, t, sh, MLLinOpT<MF>::BCMode::Homogeneous, MLLinOpT<MF>::StateMode::Correction);
+        Lp.apply(amrlev, mglev, t, r, MLLinOpT<MF>::BCMode::Homogeneous, MLLinOpT<MF>::StateMode::Correction);
         Lp.normalize(amrlev, mglev, t);
         //
         // This is a little funky.  I want to elide one of the reductions
@@ -201,8 +198,8 @@ MLCGSolverT<MF>::solve_bicgstab (MF& sol, const MF& rhs, RT eps_rel, RT eps_abs)
         {
             ret = 3; break;
         }
-        MF::Saxpy(sol, omega, sh, 0, 0, ncomp, nghost); // sol += omega * sh
-        MF::Saxpy(r,  -omega, t,  0, 0, ncomp, nghost); // r += -omega * t
+        MF::Saxpy(sol, omega, r, 0, 0, ncomp, nghost); // sol += omega * r
+        MF::Saxpy(r,  -omega, t, 0, 0, ncomp, nghost); // r += -omega * t
 
         rnorm = norm_inf(r);