[WebAssembly] Fix p2align in assembler.

Summary:
- Match the syntax output by InstPrinter.
- Fix it always emitting 0 for align. Had to work around fact that
  opcode is not available for GetDefaultP2Align while parsing.
- Updated tests that were erroneously happy with a p2align=0

Fixes https://bugs.llvm.org/show_bug.cgi?id=40752

Reviewers: aheejin, sbc100

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63633

llvm-svn: 364570