Revert r302547 ([mips] Impose a threshold for coercion of aggregates)

Reverting
  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.
as it broke clang-with-lto-ubuntu builder.

llvm-svn: 302555