Simplify ira_setup_alts

Simplify ira_setup_alts

ira_setup_alts has its own code to calculate the start of the
constraint string for each operand/alternative combination,
but preprocess_constraints now provides that information in (almost)
constant time for non-asm instructions.  Using it here should speed
up the common case at the cost of potentially slowing down the handling
of asm statements.

The real reason for doing this is that a later patch wants to use
more of the operand_alternative information.

2019-07-01  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* ira.c (ira_setup_alts): Use preprocess_constraints to get the
	constraint string for each operand/alternative combo.  Only handle
	'%' at the start of constraint strings, and look for it outside
	the main loop.

From-SVN: r272848