[Bug target/87507] IRA unnecessarily uses non-volatile registers during register assignment

[bergner at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87507

--- Comment #3 from Peter Bergner <bergner at gcc dot gnu.org> ---
So the register pair r7,r8 is free to assign to TImode pseudo 126 in
assign_hard_reg(), but its cost is 1000, while the cost of r30,r31 somehow
looks to be zero, so we choose it instead!?!?  Not sure how r30,r31 incorrectly
got a cost of zero.