Increment/decrement scalar register by (scaled) element count given by
predicate pattern, e.g. 'incw x0, all, mul #4'.
Reviewers: rengolin, fhahn, SjoerdMeijer, samparker, javed.absar
Reviewed By: SjoerdMeijer
Differential Revision: https://reviews.llvm.org/D47713
llvm-svn: 334838