[RegisterBankInfo] Keep valid pointers for PartialMappings.

Previously we were using the address of the unique instance of a partial
mapping in the related map to access this instance. However, when the
map grows, the whole set of instances may be moved elsewhere and the
previous addresses are not valid anymore.

Instead, keep the address of the unique heap allocated instance of a
partial mapping.

Note: I did not see any actual bugs for that problem as the number of
partial mappings dynamically allocated is small (<= 4).

llvm-svn: 282323