TargetLowering: Add getVectorIdxTy() function v2

This virtual function can be implemented by targets to specify the type
to use for the index operand of INSERT_VECTOR_ELT, EXTRACT_VECTOR_ELT,
INSERT_SUBVECTOR, EXTRACT_SUBVECTOR.  The default implementation returns
the result from TargetLowering::getPointerTy()

The previous code was using TargetLowering::getPointerTy() for vector
indices, because this is guaranteed to be legal on all targets.  However,
using TargetLowering::getPointerTy() can be a problem for targets with
pointer sizes that differ across address spaces.  On such targets,
when vectors need to be loaded or stored to an address space other than the
default 'zero' address space (which is the address space assumed by
TargetLowering::getPointerTy()), having an index that
is a different size than the pointer can lead to inefficient
pointer calculations, (e.g. 64-bit adds for a 32-bit address space).

There is no intended functionality change with this patch.