Reland: [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

with minor changes (use regexp instead of the hardcoded values) to the test.

llvm-svn: 302670