[Hexagon] Do not reduce load size for globals in small-data

Small-data (i.e. GP-relative) loads and stores allow 16-bit scaled
offset. For a load of a value of type T, the small-data area is
equivalent to an array "T sdata[65536]". This implies that objects
of smaller sizes need to be closer to the beginning of sdata,
while larger objects may be farther away, or otherwise the offset
may be insufficient to reach it. Similarly, an object of a larger
size should not be accessed via a load of a smaller size.

llvm-svn: 345975