[SystemZ] Improve decoding in case of instructions with four register operands.

Since z13, the max group size will be 2 if any μop has more than 3 register
sources.

This has been ignored sofar in the SystemZHazardRecognizer, but is now
handled by recognizing those instructions and adjusting the tracking of
decoding and the cost heuristic for grouping.

Review: Ulrich Weigand
https://reviews.llvm.org/D49847

llvm-svn: 338368