[CodeGen] add rotate builtins

This exposes the LLVM funnel shift intrinsics as more familiar bit rotation functions in clang 
(when both halves of a funnel shift are the same value, it's a rotate).

We're free to name these as we want because we're not copying gcc, but if there's some other 
existing art (eg, the microsoft ops that are modified in this patch) that we want to replicate, 
we can change the names.

The funnel shift intrinsics were added here:
D49242

With improved codegen in:
rL337966
rL339359

And basic IR optimization added in:
rL338218
rL340022

...so these are expected to produce asm output that's equal or better to the multi-instruction 
alternatives using primitive C/IR ops.

In the motivating loop example from PR37387:
https://bugs.llvm.org/show_bug.cgi?id=37387#c7
...we get the expected 'rolq' x86 instructions if we substitute the rotate builtin into the source.

Differential Revision: https://reviews.llvm.org/D50924

llvm-svn: 340135