PR12716: PPC crashes on vector compare

Vector compare using altivec 'vcmpxxx' instructions have as third argument
a vector register instead of CR one, different from integer and float-point
compares. This leads to a failure in code generation, where 'SelectSETCC'
expects a DAG with a CR register and gets vector register instead.

This patch changes the behavior by just returning a DAG with the 
vector compare instruction based on the type. The patch also adds a testcase
for all vector types llvm defines.

It also included a fix on signed 5-bits predicates printing, where
signed values were not handled correctly as signed (char are unsigned by
default for PowerPC). This generates 'vspltisw' (vector splat)
instruction with SIM out of range.

llvm-svn: 165419