[globalisel][aarch64] Fix unintended assumptions about PartialMappingIdx. NFC.

Summary:
This is NFC but prevents assertions when PartialMappingIdx is tablegen-erated.
The assumptions were:
1) FirstGPR is 0
2) FirstGPR is the first of the First* enumerators.

GPR32 is changed to 1 to demonstrate that assumption #1 is fixed. #2 will
be covered by a subsequent patch that tablegen-erates information and swaps
the order of GPR and FPR as a side effect.

Depends on D27336

Reviewers: ab, t.p.northover, qcolombet

Subscribers: aemerson, rengolin, vkalintiris, dberris, rovka, llvm-commits

Differential Revision: https://reviews.llvm.org/D27337

llvm-svn: 288812