[PowerPC][HTM]Fix $zero is not a GPRC register for builtin_ttest

This was found during HTM cleanup.
Adding a test for builtin_ttest would expose following issue.

*** Bad machine code: Illegal physical register for instruction ***
 - function:    test10
 - basic block: %bb.0 entry (0xf0e57497b58)
 - instruction: %5:crrc0 = TABORTWCI 0, $zero, 0
 - operand 2:   $zero
  $zero is not a GPRC register.
LLVM ERROR: Found 1 machine code errors.

Differential Revision: https://reviews.llvm.org/D63079

llvm-svn: 362974