cmd/compile,cmd/asm: fix ppc64 usage of BI argument of BC opcode

cmd/compile,cmd/asm: fix ppc64 usage of BI argument of BC opcode

Avoid coercing the CR bit into a GPR register type argument, and
move the existing usage to CRx_y register types. And, update the
compiler usage to this. This transformation is done internally,
so it should not alter existing assembly code.

Likewise, add assembly tests for all optab entries of BC/BR. This
found some cases which were not possible to realize with handwritten
asm, or assemble to something very unexpected if generated by the
compiler. The following optab entries are removed, and the cases
simplified or removed:

{as: ABR, a3: C_SCON, a6: C_LR, type_: 18, size: 4}

  This existed only to pass the BH hint to JMP (LR) from compiler
  generated code. It cannot be matched with asm. Instead, add and
  support 4-operand form "BC{,L} $BO, $BI, $BH, (LR)".

{as: ABR, a1: C_REG, a6: C_CTR, type_: 18, size: 4}

  Could be used like  "BR R1, (CTR)", but always compiles to bctr
  irrespective of arg 1. Any usage should be rewritten as "JMP (CTR)",
  or rewritten if this was not the intended behavior.

{as: ABR, a6: C_ZOREG, type_: 15, size: 8}:
{as: ABC, a6: C_ZOREG, type_: 15, size: 8},

  Not reachable: 0(reg) is coerced to reg in assembler frontend.

{as: ABC, a2: C_REG, a6: C_LR, type_: 18, size: 4}
{as: ABC, a2: C_REG, a6: C_CTR, type_: 18, size: 4}

  Only usable from the compiler. However, the compiler does not
  generate this form today. Without a BO operand (usually in a1), it
  is not clear what this should assemble to.

Change-Id: I1b5151f884a5877e4a610e6fd41261e8e64c5454
Reviewed-on: https://go-review.googlesource.com/c/go/+/357775
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Run-TryBot: Paul Murphy <murp@ibm.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>