[55/77] Use scalar_int_mode in simplify_const_unary_operation

[55/77] Use scalar_int_mode in simplify_const_unary_operation

The main scalar integer block in simplify_const_unary_operation
had the condition:

  if (CONST_SCALAR_INT_P (op) && width > 0)

where "width > 0" was a roundabout way of testing != VOIDmode.
This patch replaces it with a check for a scalar_int_mode instead.
It also uses the number of bits in the input rather than the output
mode to determine the result of a "count ... bits in zero" operation.
(At the momemnt these modes have to be the same, but it still seems
conceptually wrong to use the number of bits in the output mode.)

The handling of float->integer ops also checked "width > 0",
but this was redundant with the earlier check for MODE_INT.

2017-08-30  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* simplify-rtx.c (simplify_const_unary_operation): Use
	is_a <scalar_int_mode> instead of checking for a nonzero
	precision.  Forcibly convert op_mode to a scalar_int_mode
	in that case.  More clearly differentiate the operand and
	result modes and use the former when deciding what the value
	of a count-bits operation should be.  Use is_int_mode instead
	of checking for a MODE_INT.  Remove redundant check for whether
	this mode has a zero precision.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r251507