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[CVP] Remove a masking operation if range information implies it's a noop

Authored by reames on Oct 10 2019, 8:48 PM.

Description

[CVP] Remove a masking operation if range information implies it's a noop

This is really a known bits style transformation, but known bits isn't context sensitive. The particular case which comes up happens to involve a range which allows range based reasoning to eliminate the mask pattern, so handle that case specifically in CVP.

InstCombine likes to generate the mask-by-low-bits pattern when widening an arithmetic expression which includes a zext in the middle.

Differential Revision: https://reviews.llvm.org/D68811

llvm-svn: 374506