[AArch64][GlobalISel] Add manual selection support for G_ZEXTLOADs to s64.

We already get support for G_ZEXTLOAD to s32 from the importer, but it can't
deal with the SUBREG_TO_REG in the pattern. Tweaking the existing manual
selection code for G_LOAD to handle an additional SUBREG_TO_REG when dealing
with G_ZEXTLOAD isn't much work.

Also add tests to check the imported pattern selections to s32 work.

llvm-svn: 362681