panfrost/mdg: Use appropriate sizes for global loads/stores

When always using 128-bit operations, an access at the end of an SSBO could spill over to the next page and cause a GPU fault. To avoid this, pick the smallest instruction that fits.

The if ladder might need extending for sizes of less than 32 bits, but those currently fail in other parts of the compiler so are untested.

Also rename the load/store operations to explicitly state the bit size.

Fixes some of the faults for #4028 (closed).