rtl-optimization/110587 - speedup find_hard_regno_for_1

The following applies a micro-optimization to find_hard_regno_for_1,
re-ordering the check so we can easily jump-thread by using an else.
This reduces the time spent in this function by 15% for the testcase
in the PR.

	PR rtl-optimization/110587
	* lra-assigns.cc (find_hard_regno_for_1): Re-order checks.