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[JumpThreading] Ignore free instructions

Authored by nikic on Sep 22 2021, 12:34 PM.

Description

[JumpThreading] Ignore free instructions

This is basically D108837 but for jump threading. Free instructions
should be ignored for the threading decision. JumpThreading already
skips some free instructions (like pointer bitcasts), but does not
skip various free intrinsics -- in fact, it currently gives them a
fairly large cost of 2.

Differential Revision: https://reviews.llvm.org/D110290