asm-llvm: Optimize parseExprOp, which profiling says is a hot spot.
asm-llvm: Improve syntax error messages.
asm-llvm: branch order optimizations.

Rearrange some branches in the parser so that most-frequently-taken
branches come first.
Add some extra parentheses to example-asm-parens test case.

Prompted by code coverage results: I noticed there was parentheses-handling
code that we weren't yet exercising.