[WebAssembly] Implement prolog/epilog insertion and FrameIndex elimination

[WebAssembly]  Implement prolog/epilog insertion and FrameIndex elimination

Summary:
Use the SP32 physical register as the base for FrameIndex
lowering. Update it and the __stack_pointer global var in the prolog and
epilog. Extend the mapping of virtual registers to wasm locals to
include the physical registers.

Rather than modify the target-independent PrologEpilogInserter (which
asserts that there are no virtual registers left) include a
slightly-modified copy for Wasm that does not have this assertion and
only clears the virtual registers if scavenging was needed (which of
course it isn't for wasm).

Differential Revision: http://reviews.llvm.org/D15344

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@255392 91177308-0d34-0410-b5e6-96231b3b80d8