[SimpleLoopUnswitch] Enable freezing of conditions by default.

This fixes a series of mis-compiles by SimpleLoopUnswitch.

My measurements showed no performance regression with -O3 on AArch64
in SPEC2006, SPEC2017 and a set of internal benchmarks.

Fixes #50387, #50430

Depends on D124251.

Reviewed By: nikic, aqjune

Differential Revision: https://reviews.llvm.org/D124252