As Jaiwei noted, many (all?) of the rvv/base/spill tests started failing 
after the introduction of shrink-wrapping.


The core issue is we're expecting the frame to have a constant size, but 
it doesn't.  So when using the to_constant method we abort.


The safest thing to do is to set no shrink-wrapping components when the 
frame size is not fixed.  We might be able to do better later -- iff we 
know the offset to the GPRs/FPRs is fixed and fits into the appropriate 
number of bits.


Bootstrapped and regression tested (C-only) on riscv64-linux-gnu.  As 
expected, it fixes a bucketload of failures in rvv/base/spill-*.c.


Installed on the trunk,

Jeff