[tablegen] Extract portions of AsmMatcherEmitter for re-use by another generator. NFC.

Summary:
This change is preparation for a change that will allow targets to verify that the instructions
they emit meet the predicates they specify. This is useful to ensure that C++
legalization/lowering/instruction-selection doesn't incorrectly select code for a different
subtarget than intended. Such cases are not caught by the integrated assembler when emitting
instructions directly to an object file.

Reviewers: qcolombet

Subscribers: qcolombet, beanz, mgorny, llvm-commits, modocache

Differential Revision: https://reviews.llvm.org/D25614

llvm-svn: 286945