[PowerPC] Support asm parsing for bc[l][a][+-] mnemonics

[PowerPC] Support asm parsing for bc[l][a][+-] mnemonics

PowerPC assembly code in the wild, so it seems, has things like this:

  bc+     12, 28, .L9

This is a bit odd because the '+' here becomes part of the BO field, and the BO
field is otherwise the first operand. Nevertheless, the ISA specification does
clearly say that the +- hint syntax applies to all conditional-branch mnemonics
(that test either CTR or a condition register, although not the forms which
check both), both basic and extended, so this is supposed to be valid.

This introduces some asm-parser-only definitions which take only the upper
three bits from the specified BO value, and the lower two bits are implied by
the +- suffix (via some associated aliases).

Fixes PR23646.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@280571 91177308-0d34-0410-b5e6-96231b3b80d8