[InstCombine] Remove side effect of replaced constrained intrinsics

If a constrained intrinsic call was replaced by some value, it was not
removed in some cases. The dangling instruction resulted in useless
instructions executed in runtime. It happened because constrained
intrinsics usually have side effect, it is used to model the interaction
with floating-point environment. In some cases it is correct behavior
but often the side effect is actually absent or can be ignored.

This change adds specific treatment of constrained intrinsics so that
their side effect can be removed if it actually absents.

Differential Revision: https://reviews.llvm.org/D118426