[DAGCombiner] narrow truncated binops

The motivating case for this is shown in:
https://bugs.llvm.org/show_bug.cgi?id=32023
and the corresponding rot16.ll regression tests.

Because x86 scalar shift amounts are i8 values, we can end up with trunc-binop-trunc 
sequences that don't get folded in IR.

As the TODO comments suggest, there will be regressions if we extend this (for x86, 
we mostly seem to be missing LEA opportunities, but there are likely vector folds 
missing too). I think those should be considered existing bugs because this is the 
same transform that we do as an IR canonicalization in instcombine. We just need 
more tests to make those visible independent of this patch.

Differential Revision: https://reviews.llvm.org/D54640

llvm-svn: 347917