There's a costly signed 64-bit division in rtx_cost on x86 as well as
any other target where UNITS_PER_WORD expands to TARGET_64BIT ?  8 : 4.
It's also evident that rtx_cost does redundant work for a SET.

Obviously the variable named 'factor' rarely exceeds 1, so in the
majority of cases it can be computed with a well-predictable branch
rather than a division.

This patch makes rtx_cost do the division only in case mode is wider
than UNITS_PER_WORD, and also moves a test for a SET up front to avoid
redundancy.
No functional change.

	* rtlanal.c (rtx_cost): Handle a SET up front. Avoid division if
	the mode is not wider than UNITS_PER_WORD.