Revert r152105: "Use uint16_t to store indices into string table"

This patch limited the concatenated register names to 64K which meant
that the total number of registers was many times less than 64K.

If any compilers actually enforce the 64K limit on string literals, and
it turns out to be a problem, we should fix that problem by not using
long string literals.

llvm-svn: 152816