rtl-optimization/54052 - RTL SSA PHI insertion compile-time hog

The following tries to address the PHI insertion compile-time hog in
RTL fwprop observed with the PR54052 testcase where the loop computing
the "unfiltered" set of variables possibly needing PHI nodes for each
block exhibits quadratic compile-time and memory-use.

It does so by pruning the local DEFs with LR_OUT of the block, removing
regs that can never be LR_IN (defined by this block) in the dominance
frontier.

	PR rtl-optimization/54052
	* rtl-ssa/blocks.cc (function_info::place_phis): Filter
	local defs by LR_OUT.