[PR102627] Use at least natural mode during splitting hard reg live range

[PR102627] Use at least natural mode during splitting hard reg live range

In the PR test case SImode was used to split live range of cx on x86-64
because it was the biggest mode for this hard reg in the function.  But
all 64-bits of cx contain structure members.  We need always to use at least
natural mode of hard reg in splitting to fix this problem.

gcc/ChangeLog:

	PR rtl-optimization/102627
	* lra-constraints.c (split_reg): Use at least natural mode of hard reg.

gcc/testsuite/ChangeLog:

	PR rtl-optimization/102627
	* gcc.target/i386/pr102627.c: New test.