[RFA:] Fix uninitialized-bug in gcc/bitmap.c, spotted by valgrind.

[Joern Rennecke]

Doesn't c in bitmap_equal_p need the same initialization?

How about tmp_head in flow.c:calculate_global_regs_live?
diff_head in flow.c:init_propagate_block_info ?
tmp_head in ifcvt.c:dead_or_predicable ?
	
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