[FPEnv][ARM] Don't call mutateStrictFPToFP when lowering

mutateStrictFPToFP can delete the node and replace it with another with the same
value which can later cause problems, and returning the result of
mutateStrictFPToFP doesn't work because SelectionDAGLegalize expects that the
returned value has the same number of results as the original. Instead handle
things by doing the mutation manually.

Differential Revision: https://reviews.llvm.org/D74726