cmd/compile/internal/gc: cleaning lex.go

cmd/compile/internal/gc: cleaning lex.go

Cleaning along the way:
-convert variable types from int to bool
-remove unnecessary functions
-remove unnecessary type conversion
-remove unnecessary variable declarations
-transform struct{string,string} with lookup to map[string]string

This change passes go build -toolexec 'toolstash -cmp' -a std.

Change-Id: I259728fe4afd7f23b67f08fab856ce0abee57b21
Reviewed-on: https://go-review.googlesource.com/14435
Reviewed-by: Ian Lance Taylor <iant@golang.org>