[AArch64][GlobalISel] Add AArch64::G_DUPLANE[X] opcodes for lane duplicates.

These were previously handled by pattern matching shuffles in the selector, but
adding a new opcode and making it equivalent to the AArch64duplane SDAG node
allows us to select more patterns, like lane indexed FMLAs (patch adding a test
for that will be committed later).

The pattern matching code has been simply moved to postlegalize lowering.

Differential Revision: https://reviews.llvm.org/D90820