[ARM] Fix over-alignment in arguments that are HA of 128-bit vectors

Code in `CC_ARM_AAPCS_Custom_Aggregate()` is responsible for handling
homogeneous aggregates for `CC_ARM_AAPCS_VFP`. When an aggregate ends up
fully on stack, the function tries to pack all resulting items of the
aggregate as tightly as possible according to AAPCS.

Once the first item was laid out, the alignment used for consecutive
items was the size of one item. This logic went wrong for 128-bit
vectors because their alignment is normally only 64 bits, and so could
result in inserting unexpected padding between the first and second
element.

The patch fixes the problem by updating the alignment with the item size
only if this results in reducing it.

Differential Revision: https://reviews.llvm.org/D49720

llvm-svn: 338233