[EarlyIfCvt] Don't if-convert if condition has only loop-invariant ops.

This patch adds a heuristic to skip if-conversion if the condition has a
high chance of being predictable.

If the condition is in a loop, consider it predictable if the condition
itself or all its operands are loop-invariant. E.g. this considers a load
from a loop-invariant address predictable; we were unable to prove that it
doesn't alias any of the memory-writes in the loop, but it is likely to
read to same value multiple times.

This is a relatively crude heuristic, but it helps to prevent excessive
if-conversion in multiple workloads in practice.

Reviewed By: apostolakis

Differential Revision: https://reviews.llvm.org/D141639