[globalisel][tablegen] Add support for (set $dst, 1) and test X86's OptForSize predicate.

Summary:
It's rare but a small number of patterns use IntInit's at the root of the match.
On X86, one such rule is enabled by the OptForSize predicate and causes the
compiler to use the smaller:
	%0 = MOV32r1
instead of the usual:
	%0 = MOV32ri 1

This patch adds support for matching IntInit's at the root and uses this as a
test case for the optsize attribute that was implemented in r301750

Reviewers: qcolombet, ab, t.p.northover, rovka, kristof.beyls, aditya_nandakumar

Reviewed By: qcolombet

Subscribers: igorb, llvm-commits

Differential Revision: https://reviews.llvm.org/D32791

llvm-svn: 303678