LRA: For clobbered regs use operand mode instead of the biggest mode

LRA is too conservative in calculation of conflicts with clobbered regs by
using the biggest access mode.  This results in failure of possible reg
coalescing and worse code.  This patch solves the problem.

        PR rtl-optimization/108999

gcc/ChangeLog:

	* lra-constraints.cc (process_alt_operands): Use operand modes for
	clobbered regs instead of the biggest access mode.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/pr108999.c: New.