[LICM] Don't try to constant fold instructions

This was introduced in 030f0202 as
an alleged compile-time optimization. In reality, trying to constant
fold instructions is more expensive than just hoisting them. In a
standard pipeline, LICM tends to run either after a run of
LoopInstSimplify or InstCombine, so LICM doesn't really see constant
foldable instructions in the first place, and the attempted fold
is futile.

This makes for a very minor compile-time improvement.

Differential Revision: https://reviews.llvm.org/D149134