[X86] combineX86ShufflesConstants - constant fold from target shuffles unless optsize = true

Currently we only constant fold target shuffles if any of the sources has one use, or it would remove a variable shuffle mask - the aim being to avoid constant pool bloat.

This patch proposes we should constant fold by default and only limit this if optsize is enabled - I've added a basic test for this in vector-mul.ll (the pmuludq case is by far the most common), I can add other specific test cases if people need them.

This should permit further constant folding, break some instruction dependencies and help reduce shuffle port pressure.

Differential Revision: https://reviews.llvm.org/D113748