shader/memory: Implement LDG.U8 and unaligned U8 loads

LDG can load single bytes instead of full integers or packs of integers.
These have the advantage of loading bytes that are not aligned to 4
bytes.

To emulate these this commit gets the byte being referenced (by doing
"address & 3" and then using that to extract the byte from the loaded
integer:

result = bitfieldExtract(loaded_integer, (address % 4) * 8, 8)