Fix flakiness in fp16-promote.ll

Summary:
In the f16-promote test, make the checks for native conversion instructions
similar to the libcall checks:
- Remove hard coded register names
- Do not check exact instruction sequences.

This fixes test flakiness due to non-determinism in instruction
scheduling and register allocation.  I also fixed a few minor things in
the CHECK-LIBCALL checks.

I'll try to find a way to check that unnecessary loads, stores, or
conversions don't happen.

Reviewers: mzolotukhin, srhines, ab

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9112

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235363 91177308-0d34-0410-b5e6-96231b3b80d8