[Mips] Remove custom "original type" handling (#154082)
Replace Mips custom logic for retaining information about original types
in calling convention lowering by directly querying the OrigTy that is
now available.
There is one change in behavior here: If the return type is a struct
containing fp128 plus additional members, the result is now different,
as we no longer special case to a single fp128 member. I believe this is
fine, because this is a fake ABI anyway: Such cases should actually use
sret, and as such are a frontend responsibility, and Clang will indeed
emit these as sret, not as a return value struct. So this only impacts
manually written IR tests.
