VECTOR_SHUFFLEs to REG_SEQUENCE instructions.  The standard ISD::BUILD_VECTOR
node corresponds closely to REG_SEQUENCE but I couldn't use it here because
its operands do not get legalized.  That is pretty awful, but I guess it
makes sense for other targets.  Instead, I have added an ARM-specific version
of BUILD_VECTOR that will have its operands properly legalized.
This fixes the rest of Radar 7872877.

llvm-svn: 105439