[FPEnv][EarlyCSE] Support for CSE when exception behavior is "ignore" or...

[FPEnv][EarlyCSE] Support for CSE when exception behavior is "ignore" or "maytrap" and the rounding mode is known.

Previously we would only CSE constrained FP intrinsics in the default
floating point environment. Exception behavior of "strict" is still not
allowed since we are not allowed to remove any traps in that case.

There are no restrictions on CSE across function calls inside a function.

Differential Revision: https://reviews.llvm.org/D112256