Fix expand_single_bit_test for big-endian

Fix expand_single_bit_test for big-endian

I had thought extract_bit_field bitpos argument was the shifted position
and not the bitposition like BIT_FIELD_REF so I had removed the code which
would use the correct bitposition for BYTES_BIG_ENDIAN.

Committed as obvious; I checked big-endian MIPS to make sure we are now
producing the correct code.

gcc/ChangeLog:

	* expr.cc (expand_single_bit_test): Correct bitpos for big-endian.