[Verifier] Move some atomicrmw/cmpxchg checks to instruction creation

These checks already exist as asserts when creating the corresponding
instruction. Anybody creating these instructions already need to take
care to not break these checks.

Move the checks for success/failure ordering in cmpxchg from the
verifier to the LLParser and BitcodeReader plus an assert.

Add some tests for cmpxchg ordering. The .bc files are created from the
.ll files with an llvm-as with these checks disabled.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D102803