Re-commit: [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.

The previous commit had an uninitialized variable that caused the incoming
argument region to have undefined size. This has been fixed.

Reviewers: sdardis

Subscribers: dsanders, llvm-commits, sdardis

Differential Revision: https://reviews.llvm.org/D22680

llvm-svn: 277136