[mips][fastisel] Handle 0-4 arguments without SelectionDAG.
Summary:
Implements fastLowerArguments() to avoid the need to fall back on
SelectionDAG for 0-4 argument functions that don't do tricky things like
passing double in a pair of i32's.
This allows us to move all except one test to -fast-isel-abort=3. The
remaining one has function prototypes of the form 'i32 (i32, double, double)'
which requires floats to be passed in GPR's.
Reviewers: sdardis
Subscribers: dsanders, llvm-commits, sdardis
Differential Revision: https://reviews.llvm.org/D22680
llvm-svn: 276982