Use partial_subreg_p in curr_insn_transform

Use partial_subreg_p in curr_insn_transform

Use partial_subreg_p in code that was added since the initial patch
that introduced this function.

2018-01-03  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* lra-constraints.c (curr_insn_transform): Use partial_subreg_p.

From-SVN: r256194