[AArch64][GlobalISel] Fix sub-64b stack parameter passing on Darwin.

A previous bug fix for varargs introduced a regression where we would
incorrectly widen some stores to memory when passing i8/i16 parameters on the
stack. This didn't show up seemingly because it only happens when there is
no signext/zeroext parameter attribute, which I think for Darwin clang adds.

Swift however seems to be a different story, and a plain anyext on the parameter
triggered the bug.

To fix this, I've added a new ValueHandler::assignValueToAddress type override
which lets us distiguish between varargs and fixed args (we still need this
widening behaviour for varargs to fix the original bug in 2018).

rdar://61353552