[InstCombine] Inefficient pattern for high-bits checking 2 (PR38708)

[InstCombine] Inefficient pattern for high-bits checking 2 (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..)

More complicated, canonical pattern:
https://rise4fun.com/Alive/uhA

We do need to have two `switch()`'es like this,
to not mismatch the swappable predicates.

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/D52001

llvm-svn: 342173