[flang] Propagate fastmath flags during intrinsics simplification.

In general, the meaning of fastmath flags on a call during inlining
is that the call's operation flags must be ignored. For user functions
that means that the fastmath flags used for the function definition
override any call site's fastmath flags. For intrinsic functions
we can use the call site's fastmath flags, but we have to make sure
that the call sites with different flags produce/use different
simplified versions of the same intrinsic function.

Differential Revision: https://reviews.llvm.org/D138048