[Hexagon] Break up vectors into HVX-sized chunks in HvxIdioms

This will allow recognizing Q.31 multiplications on vectors that are
multiplies of HVX vectors. At the moment this comes at the expense of
Q.15 multiplications, which now are handled as 32-bit multiplications
with shifts.
In the longer term this will likely be replaced by a different scheme
of "legalizing" vectors, which is necessary for idiom recognition, at
least where using direct HVX instrinsics is desired.