Implement remaining straightforward vasm ops in llvm backend

Summary: This is a followup to D1683817. We now emit llvm code for all the
straightforward vasm instructions, where "straightforward" is defined as not
relating to service requests, weird stub calling conventions, floating point
comparisons, and maybe a few others I'm forgetting. I also fixed a few other
issues that were blocking llvm codegen for a large number of tests. Things
worth pointing out:

- code-gen-x64 no longer directly emits shlq and sarq. Like idiv, these
  instructions have a hardcoded operand (%cl).
- Don't use CC_NS for a refcount comparison where CC_GE will do the same
  thing.
- Put the syncpoint for NativeCall in its vcall rather than emitting a
  syncpoint instruction directly.

Reviewed By: @jasone

Differential Revision: D1686955

Signature: t1:1686955:1416330185:9940240dcd4a24b066c2dfa8f37de570663630b3