TableGen/DAGPatterns: Allow bit constants in addition to int constants

Summary:
Implicit casting is a simple quality of life improvement.

Change-Id: I3d2b31b8b8f12cbb1e84f691e359fa713a9c4b42

Reviewers: tra, simon_tatham, craig.topper, MartinO, arsenm

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D47432

llvm-svn: 333904