[WebAssembly] Tidy up `let` statements in .td files (NFC)

Summary:
- Delete {} for one-line `let` statements
- Don't indent within `let` blocks
- Add comments after `let` block's closing braces

Reviewers: tlively

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57730

llvm-svn: 353248