[GlobalISel][NFC] Factor out common target code from GlobalISelEmitterTests

Factor out commonly-used target code from the GlobalISelEmitter tests into
a GlobalISelEmitterCommon.td file. This is tested by the original
GlobalISelEmitter.td test.

This reduces the amount of boilerplate code necessary for tests like this.

Differential Revision: https://reviews.llvm.org/D65777

llvm-svn: 368757