[RISC-V] Clean up code gen for floating comparisons

[tomeksowi]

There are ways to generate better code for equals-or-unordered and not-equals-and-ordered. However, there are no IL opcodes for them (ceq.un, beq.un, bne) so these code gens can't happen.

As a bonus, flip the comparison result with one instruction.

Part of #84834, cc @dotnet/samsung

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[risc-vv]

41d470b is being scheduled for building and testing

GIT: 41d470bf4390b89f83968adb94cd90371be05ecb
REPO: dotnet/runtime
BRANCH: main

Release-build FAILED

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[risc-vv]

a711137 is being scheduled for building and testing

GIT: a711137381bf85fbcd063a60990604c2f9ffd2e4
REPO: dotnet/runtime
BRANCH: main

Release-build FAILED

buildinfo.json

Compilation failed during core build```

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