[X86][SSE] Generalize shuffle(HORIZOP,HORIZOP) -> HORIZOP combine

Our existing combine allows to merge the shuffle of 2 similar 64-bit wide 'horizontal ops' (HADD/PACK/etc.) if the shuffle was a UNPCK/MOVSD.

This patch generalizes this to decode any target shuffle mask that can be widened to a 128-bit repeating v2*64 mask, which helps us catch PBLENDW/PBLENDD cases.