[SelectionDAG] Use correct boolean representation in FoldConstantArithmetic

The description of SETCC says
  /// SetCC operator - This evaluates to a true value iff the condition is
  /// true.  If the result value type is not i1 then the high bits conform
  /// to getBooleanContents.

Without this patch, we sign extended the i1 to the used larger type
regardless of getBooleanContents. This resulted in miscompiles, as
shown in the attached testcase that ended up returning -1 instead of
1 when using -mattr=+v.

Fixes https://github.com/llvm/llvm-project/issues/55168

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D124618