Reland "[WebAssembly] More uses of uint8_t for single byte values"

Summary:
Original change was D43991 (rL326541) and was reverted by rL326571 and
rL326572. This adds also the necessary MCCodeEmitter patch.

Reviewers: sbc100

Subscribers: jfb, dschuff, sbc100, jgravelle-google, sunfish, llvm-commits, ncw

Differential Revision: https://reviews.llvm.org/D44034

llvm-svn: 326614