[Hexagon] Improve handling of 32-bit mulh/mul_lohi on HVX

[Hexagon] Improve handling of 32-bit mulh/mul_lohi on HVX

Handle MULH[US] by normalizing them into newly invented nodes
HexagonISD::(S|U|US)MUL_LOHI. On HVX v60, if only the high part of
SMUL_LOHI is used, use the original MULHS expansion. In all other
cases, expand the full product.
On HVX v62, always expand the full product.

Introduce Hexagon-specific LLVM IR intrinsics for 32x32 multiplication
returning low/high parts.