VAX: Remove `c' operand format specifier overload

VAX: Remove `c' operand format specifier overload

The `c' operand format specifier is handled directly by the middle end
in `output_asm_insn':

   %cN means require operand N to be a constant
      and print the constant expression with no punctuation.

however it resorts to the target for constants that are not valid
addresses:

	    else if (letter == 'c')
	      {
		if (CONSTANT_ADDRESS_P (operands[opnum]))
		  output_addr_const (asm_out_file, operands[opnum]);
		else
		  output_operand (operands[opnum], 'c');
	      }

The VAX backend expects the fallback never to happen and overloads `c'
with the branch condition code.  This is confusing however and it is not
like we are short of letters, so instead make the branch condition code
use `k', and then for consistency make `K' the reverse branch condition
code format specifier.  This is safe to do as we provide no means to use
a computed branch condition code in user `asm'.

	gcc/
	* config/vax/vax.c (print_operand): Replace `c' and `C' with
	`k' and `K' respectively.
	* config/vax/vax.md (*branch, *branch_reversed): Update
	accordingly.