rtl-optimization/117467 - limit ext-dce memory use

The following puts in a hard limit on ext-dce because it might end
up requiring memory on the order of the number of basic blocks
times the number of pseudo registers.  The limiting follows what
GCSE based passes do and thus I re-use --param max-gcse-memory here.

This doesn't in any way address the implementation issues of the pass,
but it reduces the memory-use when compiling the
module_first_rk_step_part1.F90 TU from 521.wrf_r from 25GB to 1GB.

	PR rtl-optimization/117467
	PR rtl-optimization/117934
	* ext-dce.cc (ext_dce_execute): Do nothing if a memory
	allocation estimate exceeds what is allowed by
	--param max-gcse-memory.