[nvptx] Fix UB in nvptx_assemble_value

[nvptx] Fix UB in nvptx_assemble_value

When nvptx_assemble_value is called with size == 16, this bitshift runs
into UB:
...
  val &= ((unsigned  HOST_WIDE_INT)2 << (size * BITS_PER_UNIT - 1)) - 1;
...

Fix this by checking the shift amount.

Tested on nvptx.

gcc/ChangeLog:

	* config/nvptx/nvptx.c (nvptx_assemble_value): Fix undefined
	behaviour.