store the bitmap_head within the auto_bitmap

store the bitmap_head within the auto_bitmap

This gets rid of one allocation per bitmap.  Often the bitmap_head is
now on the stack, when it isn't its part of some other struct on the
heap instead of being refered to by that struct.  On 64 bit platforms
this will increase the size of such structs by 24 bytes, but its an over
all win since we don't need an 8 byte pointer pointing at the
bitmap_head.  Given that the auto_bitmap owns the bitmap_head anyway we
know there would never be a place where two auto_bitmaps would refer to
the same bitmap_head object.

gcc/ChangeLog:

2017-05-13  Trevor Saunders  <tbsaunde+gcc@tbsaunde.org>

	* bitmap.h (class auto_bitmap): Change type of m_bits to
	bitmap_head, and adjust ctor / dtor and member operators.

From-SVN: r248017