Bug 400490 s390x: Fix register allocation for VRs vs FPRs

Bug 400490 s390x: Fix register allocation for VRs vs FPRs

On s390x, if vector registers are available, they are fed to the register
allocator as if they were separate from the floating-point registers.  But
in fact the FPRs are embedded in the VRs.  So for instance, if both f3 and
v3 are allocated and used at the same time, corruption will result.

This is fixed by offering only the non-overlapping VRs, v16 to v31, to the
register allocator instead.