[DAGCombiner] convert logic-of-setcc into bit magic (PR40611)

If we're comparing some value for equality against 2 constants
and those constants have an absolute difference of just 1 bit,
then we can offset and mask off that 1 bit and reduce to a single
compare against zero:
         and/or (setcc X, C0, ne), (setcc X, C1, ne/eq) -->
         setcc ((add X, -C1), ~(C0 - C1)), 0, ne/eq


This transform is disabled by default using a TLI hook

That should be overridden for AArch64, MIPS, Sparc and possibly
others based on the asm shown in:

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353859 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed