[CodeGen] use saturating FP casts when compiling with "no-strict-float-cast-overflow"

We got an unintended consequence of the optimizer getting smarter when
compiling in a non-standard mode, and there's no good way to inhibit
those optimizations at a later stage. The test is based on an example
linked from D92270.

We allow the "no-strict-float-cast-overflow" exception to normal C
cast rules to preserve legacy code that does not expect overflowing
casts from FP to int to produce UB. See D46236 for details.

Differential Revision: https://reviews.llvm.org/D115804