[SLPVectorizer] Relax "alternate" opcode vectorisation to work with any SK_Select shuffle pattern

D47985 saw the old SK_Alternate 'alternating' shuffle mask replaced with the SK_Select mask which accepts either input operand for each lane, equivalent to a vector select with a constant condition operand.

This patch updates SLPVectorizer to make full use of this SK_Select shuffle pattern by removing the 'isOdd()' limitation.

The AArch64 regression will be fixed by D48172.

Differential Revision: https://reviews.llvm.org/D48174

llvm-svn: 335130