[FPEnv] EarlyCSE support for constrained intrinsics, default FP environment edition

EarlyCSE cannot distinguish between floating point instructions and
constrained floating point intrinsics that are marked as running in the
default FP environment. Said intrinsics are supposed to behave exactly the
same as the regular FP instructions. Teach EarlyCSE to handle them in that
case.

Differential Revision: https://reviews.llvm.org/D99962