[InstCombine] Shift amount reassociation (PR42391)

Summary:
Given pattern:
`(x shiftopcode Q) shiftopcode K`
we should rewrite it as
`x shiftopcode (Q+K)`  iff `(Q+K) u< bitwidth(x)`
This is valid for any shift, but they must be identical.

* https://rise4fun.com/Alive/9E2
* exact on both lshr => exact https://rise4fun.com/Alive/plHk
* exact on both ashr => exact https://rise4fun.com/Alive/QDAA
* nuw on both shl => nuw https://rise4fun.com/Alive/5Uk
* nsw on both shl => nsw https://rise4fun.com/Alive/0plg

Should fix [[ https://bugs.llvm.org/show_bug.cgi?id=42391 | PR42391]].

Reviewers: spatel, nikic, RKSimon

Reviewed By: nikic

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63812

llvm-svn: 364712