tree-optimization/114480 - speedup IDF compute

The testcase in the PR shows that it's worth splitting the processing
of the initial workset, which is def_blocks from the main iteration.
This reduces SSA incremental update time from 44.7s to 32.9s.  Further
changing the workset bitmap of the main iteration to a vector
speeds up things further to 23.5s for an overall nearly halving of
the SSA incremental update compile-time and an overall 12% compile-time
saving at -O1.

Using bitmap_ior in the first loop or avoiding (immediate) re-processing
of blocks in def_blocks does not make a measurable difference for the
testcase so I left this as-is.

	PR tree-optimization/114480
	* cfganal.cc (compute_idf): Split processing of the initial
	workset from the main iteration.  Use a vector for the
	workset of the main iteration.

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