[PowerPC] Fix a conversion is not considered when the ISD::BR_CC node making...

[PowerPC] Fix a conversion is not considered when the ISD::BR_CC node making the instruction selection

Summary:
 A signed comparison of i1 values produces the opposite result to an unsigned one if the condition code 
 includes less-than or greater-than. This is so because 1 is the most negative signed i1 number and the 
 most positive unsigned i1 number. The CR-logical operations used for such comparisons are non-commutative
 so for signed comparisons vs. unsigned ones, the input operands just need to be swapped.

Reviewed By: steven.zhang

Differential Revision: https://reviews.llvm.org/D54825

llvm-svn: 347831