Implement a bunch more vasm instructions in llvm backend

Summary: Nothing too interesting here. I added an srem instruction to take the
place of the 'cqo; idiv' sequence since both of those instructions have
hardcoded inputs/outputs. And we force inline freeing of locals regardless of
count to avoid dealing with the weird calling convention of the decref stub for
now. I also removed a few dead vasm instructions.

Reviewed By: @edwinsmith

Differential Revision: D1683817

Signature: t1:1683817:1416055219:a2d3cb4ddda65410eb17663d796512b124ec4895