[MIPS GlobalISel] NarrowScalar G_MUL

Narrow Scalar G_MUL for MIPS32.
Revisit NarrowScalar implementation in LegalizerHelper.
Introduce new helper function multiplyRegisters.
It performs generic multiplication of values held in multiple registers.
Generated instructions use only types NarrowTy and i1.
Destination can be same or two times size of the source.

Differential Revision: https://reviews.llvm.org/D58824

llvm-svn: 355814