[InstCombine] Inefficient pattern for high-bits checking 3 (PR38708)

[InstCombine] Inefficient pattern for high-bits checking 3 (PR38708)

Summary:
It is sometimes important to check that some newly-computed value
is non-negative and only n bits wide (where n is a variable.)
There are many ways to check that:
https://godbolt.org/z/o4RB8D
The last variant seems best?
(I'm sure there are some other variations i haven't thought of..)

The last (as far i know?) pattern, non-canonical due to the extra use.
https://godbolt.org/z/aCMsPk
https://rise4fun.com/Alive/I6f

https://bugs.llvm.org/show_bug.cgi?id=38708

Reviewers: spatel, craig.topper, RKSimon

Reviewed By: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D52062

llvm-svn: 342321