[SystemZ] Support conditional indirect sibling calls via BCR

This adds a conditional variant of CallBR instruction, CallBCR. Also,
it can be fused with integer comparisons, resulting in one of the new
C*BCall instructions.

In addition to CallBRCL limitations, this has another one: it won't
trigger if the function to call isn't already in %r1 - see f22 in the
test for an example (it's also why the loads in tests are volatile).

Author: koriakin
Differential Revision: http://reviews.llvm.org/D18928

llvm-svn: 265933