[emacs] simplify and improve keyword highlighting in tablegen-mode.el

Summary:
The keyword and type keyword matchers in tablegen-mode.el checked
for space, newline, tab, or open paren after the regular expression
that matches keywords (or type keywords, respectively). This is
unnecessary, because those regular expressions already include word
boundaries. This change removes the extra check. This also causes
"def" in "def:" to be highlighted as a keyword, which was missed
before.

Reviewers: lattner, MaskRay

Reviewed By: MaskRay

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68002

llvm-svn: 372904