fix typo in attr_fnspec::verify

fix typo in attr_fnspec::verify

Odd-numbered indices describing argument access sizes in the fnspec
string can only hold 't' or a digit, as tested in the beginning of the
case.  When checking that the size-supplying argument does not have
additional information associated with it, the test that excludes the
't' possibility looks for it at the even position in the fnspec
string.  Oops.

This might yield false positives and negatives if a function has a
fnspec in which an argument uses a 't' access-size, and ('t' - '1')
happens to be the index of an argument described in an fnspec string.
Assuming ASCII encoding, it would take a function with at least 68
arguments described in fnspec.  Still, probably worth fixing.

for  gcc/ChangeLog

	* tree-ssa-alias.c (attr_fnspec::verify): Fix index in
	non-'t'-sized arg check.