[WebAssembly] Check return value of getRegForValue in FastISel

Previously, FastISel for WebAssembly wasn't checking the return value of
`getRegForValue` in certain cases, which would generate instructions
referencing NoReg. This patch fixes this behavior.

Patch by Dominic Chen

Differential Revision: https://reviews.llvm.org/D23100

llvm-svn: 277742