[Hexagon] Fix optimize address mode pass only handle BaseImmOffset mode

This is a fix for a crash in the HexagonOptAddrMode pass that was looking
for the third operand (offset) in the following instruction that does not,
in fact, have a third operand:

  $r1 = L2_loadw_locked $r1

Additionally, this patch also adds an addrMode value to vgather pseudos
in the Hexagon backend.

Differential Revision: https://reviews.llvm.org/D117133