PTX: Fix predicate logic bug

Code such as:

%vreg100 = setcc %vreg10, -1, SETNE
brcond %vreg10, %tgt

was being incorrectly morphed into

%vreg100 = and %vreg10, 1
brcond %vreg10, %tgt

where the 'and' instruction could be eliminated since
such logic is on 1-bit types in the PTX back-end, leaving
us with just:

brcond %vreg10, %tgt

which essentially gives us inverted branch conditions.

llvm-svn: 153364