[SystemZ] Add remaining branch instructions

This patch adds assembler support for the remaining branch instructions:
the non-relative branch on count variants, and all variants of branch
on index.

The only one of those that can be readily exploited for code generation
is BRCTH (branch on count using a high 32-bit register as count).  Do
use it, however, it is necessary to also introduce a hew CHIMux pseudo
to allow comparisons of a 32-bit value agains a short immediate to go
into a high register as well (implemented via CHI/CIH).

This causes a bit of codegen changes overall, but those have proven to
be neutral (or even beneficial) in performance measurements.

llvm-svn: 288029