[RegisterBankInfo] Cache the getMinimalPhysRegClass information

TargetRegisterInfo::getMinimalPhysRegClass is actually pretty expensive
because it has to iterate over all the register classes.
Cache this information as we need and get it so that we limit its usage.
Right now, we heavily rely on it, because this is how we get the mapping
for vregs defined by copies from physreg (i.e., the one that are ABI
related).

Improve compile time by up to 10% for that pass.

NFC

llvm-svn: 315759