[mlir] add RemoveConstantIfCondition to populateOpenACCToSCFConversionPatterns

Fixes #60058  https://github.com/llvm/llvm-project/issues/60058
It hit assert when legalizePatternResult on success of ExpandIfCondition which did nothing just return success when if condition is constant.

Added RemoveConstantIfCondition to remove the if cond by getCanonicalizationPatterns.
Also remove the check for constant if cond in ExpandIfCondition and
change check ifCond to assert because only op with ifCond will need legalize in ConvertOpenACCToSCFPass

Differential Revision: https://reviews.llvm.org/D142286