[Reassociate] Enable FP reassociation via 'reassoc' and 'nsz'

Compiling with '-ffast-math' tuns on all the FastMathFlags (FMF), as
expected, and that enables FP reassociation. Only the two FMF flags
'reassoc' and 'nsz' are technically required to perform reassociation,
but disabling other unrelated FMF bits is needlessly suppressing the
optimization.

This patch fixes that needless suppression, and makes appropriate
adjustments to test-cases, fixing some outstanding TODOs in the process.

Fixes: #56483

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D129523