[InstCombine] if the condition of a select may be known via assumes, eliminate the select

This is a limited solution for PR31512:
https://llvm.org/bugs/show_bug.cgi?id=31512

The motivation is that we will need to increase usage of llvm.assume and/or metadata to solve PR28430:
https://llvm.org/bugs/show_bug.cgi?id=28430

...and this kind of simplification is needed to take advantage of that extra information.

The 'not' test case would be handled by:
https://reviews.llvm.org/D28485

Differential Revision:
https://reviews.llvm.org/D28337


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@291915 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed