[AArch64][GlobalISel] Split G_GLOBAL_VALUE into ADRP + G_ADD_LOW and optimize.

The concept of G_GLOBAL_VALUE is nice and simple, but always using it as the
representation for global var addressing until selection time creates some
problems in optimizing accesses in certain code/relocation models.

The problem comes from trying to optimize adrp -> add -> load/store sequences
in the most common "small" code model. These accesses can be optimized into an
adrp -> load with the add offset being folded into the load's immediate field.
If we try to keep all global var references as a single generic instruction
then by the time we get to the complex operand trying to match these, we end up
generating an adrp at the point of use. The real issue here is that we don't
have any form of CSE during selection, so the code size will bloat from many
redundant adrp's.

This patch custom legalizes small code mode non-GOT G_GLOBALs into target ADRP
and a new "target specific generic opcode" G_ADD_LOW. We also teach the
localizer to localize these instructions via the custom hook that was added
recently. Finally, the complex pattern for indexed loads/stores is extended to
try to fold these G_ADD_LOW instructions into the load immediate.

On -O0 CTMark, we see a 0.8% geomean code size improvement. We should also see
some minor performance improvements too.

Differential Revision: https://reviews.llvm.org/D78465