[AArch64][GISEL] Reduce likelihood of hash collisions for mappings in RegisterBankInfo (#87033)

Fixes #85209

This patch removes the truncation from `hash_code` aka `size_t` down to
`unsigned`, that currently happens on DenseMap accesses in
RegisterBankInfo. This reduces the likelihood of hash collisions, as
well as the likelihood of hitting EmptyKey or TombstoneKey, the special
key values of DenseMap. This is not the ultimate solution to the
problem, but we can do it in any case.