[AArch64][GlobalISel] Fold constants into G_GLOBAL_VALUE
This is pretty much just ports performGlobalAddressCombine from
AArch64ISelLowering. (AArch64 doesn't use the generic DAG combine for this.)
This adds a pre-legalize combine which looks for this pattern:
%g = G_GLOBAL_VALUE @x %ptr1 = G_PTR_ADD %g, cst1 %ptr2 = G_PTR_ADD %g, cst2 ... %ptrN = G_PTR_ADD %g, cstN
And then, if possible, transforms it like so:
%g = G_GLOBAL_VALUE @x %offset_g = G_PTR_ADD %g, -min(cst) %ptr1 = G_PTR_ADD %offset_g, cst1 %ptr2 = G_PTR_ADD %offset_g, cst2 ... %ptrN = G_PTR_ADD %offset_g, cstN
Where min(cst) is the smallest out of the G_PTR_ADD constants.
This means we should save at least one G_PTR_ADD.
This also updates code in the legalizer + selector which assumes that
G_GLOBAL_VALUE will never have an offset and adds/updates relevant tests.
Differential Revision: https://reviews.llvm.org/D96624