tree-optimization/113910 - huge compile time during PTA

For the testcase in PR113910 we spend a lot of time in PTA comparing
bitmaps for looking up equivalence class members.  This points to
the very weak bitmap_hash function which effectively hashes set
and a subset of not set bits.

The major problem with it is that it simply truncates the
BITMAP_WORD sized intermediate hash to hashval_t which is
unsigned int, effectively not hashing half of the bits.

This reduces the compile-time for the testcase from tens of minutes
to 42 seconds and PTA time from 99% to 46%.

	PR tree-optimization/113910
	* bitmap.cc (bitmap_hash): Mix the full element "hash" to
	the hashval_t hash.