[Hexagon] Fix PIC crash when lowering HVX vector constants (#175413)
Fix a PIC-only crash in Hexagon HVX lowering where we ended up treating
a vector-typed constant-pool reference as an address (e.g. when forming
PC-relative addresses), which triggers a type mismatch during lowering.
Build the constant-pool reference with the target pointer type instead,
then load the HVX vector from that address.
(cherry picked from commit dd63117c1a97836d2bd8856457927e3f20149b33)