[globalisel][tablegen] Simplify named operand/operator lookups and fix a...

[globalisel][tablegen] Simplify named operand/operator lookups and fix a wrong-code bug this revealed.

Summary:
Operand variable lookups are now performed by the RuleMatcher rather than
searching the whole matcher hierarchy for a match. This revealed a wrong-code
bug that currently affects ARM and X86 where patterns that use a variable more
than once in the match pattern will be imported but won't check that the
operands are identical. This can cause the tablegen-erated matcher to
accept matches that should be rejected.

Depends on D36569

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

Subscribers: aemerson, igorb, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D36618

llvm-svn: 315780