[CodeGenPrepare] Fold br(freeze(icmp x, const)) to br(icmp(freeze x, const))

Summary:
This patch helps CodeGenPrepare move freeze into the icmp when it is used by branch.
It reenables generation of efficient conditional jumps.

This is only done when at least one of icmp's operands is constant to prevent the transformation from increasing # of freeze instructions.

Performance degradation of MultiSource/Benchmarks/Ptrdist/yacr2/yacr2.test is resolved with this patch.

Checked with Alive2

Reviewers: reames, fhahn, nlopes

Reviewed By: reames

Subscribers: jdoerfert, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75859