[mips] Impose a threshold for coercion of aggregates

Modified MipsABIInfo::classifyArgumentType so that it now coerces aggregate
structures only if the size of said aggregate is less than 16/64 bytes,
depending on the ABI.

Patch by Stefan Maksimovic.

Differential Revision: https://reviews.llvm.org/D32900

llvm-svn: 302547