rs6000: powerpc suboptimal boolean test of contiguous bits [PR102239]

rs6000: powerpc suboptimal boolean test of contiguous bits [PR102239]

Add specialized version to combine two instructions from

 9: {r123:CC=cmp(r124:DI&0x600000000,0);clobber scratch;}
       REG_DEAD r124:DI
 10: pc={(r123:CC==0)?L15:pc}
      REG_DEAD r123:CC

to:

 10: {pc={(r123:DI&0x600000000==0)?L15:pc};clobber scratch;clobber %0:CC;}

then split2 will split it to one rotate dot instruction (to save one
rotate back instruction) as shifted result doesn't matter when comparing
to 0 in CCEQmode.

Bootstrapped and regression tested pass on Power 8/9/10.

gcc/ChangeLog:

	PR target/102239
	* config/rs6000/rs6000-protos.h (rs6000_is_valid_rotate_dot_mask): New
	declare.
	* config/rs6000/rs6000.c (rs6000_is_valid_rotate_dot_mask): New
	function.
	* config/rs6000/rs6000.md (*branch_anddi3_dot): New.

gcc/testsuite/ChangeLog:

	PR target/102239
	* gcc.target/powerpc/pr102239.c: New test.