Adjust mips test for recent ifcvt costing changes

MIPS ports have been failing a few tests since the change to add cost
checks in another path through the if-converter pass.

As with the other ports, these look like cases where we don't do good
costing in the MIPS port.  Someone who cares about MIPS will need to
fix this properly.

In the mean time this patch adjusts the branch cost when running the
two affected tests and skips them at -Os.  This is enough to verify
that if conversion can still happen if the costs are adjusted.

gcc/testsuite
	* gcc.target/mips/mips-ps-type-2.c: Adjust branch cost to
	encourage if-conversion.  Skip for -Os.
	* gcc.target/mips/movcc-3.c: Similarly.