Pointed out in a comment for D49754, register spilling will currently
spill SPE registers at almost any offset. However, the instructions
evstdd and evldd require a) 8-byte alignment, and b) a limit of 256
(unsigned) bytes from the base register, as the offset must fix into a
5-bit offset, which ranges from 0-31 (indexed in double-words).
This enforces the alignment in offsetMinAlign(), and enforces the spill
range check in PPCRegisterInfo::eliminateFrameIndex().
The update to the register spill test is taken partially from the test
case shown in D49754.
Additionally, pointed out by Kei Thomsen, globals will currently use
evldd/evstdd, though the offset isn't known at compile time, so may
exceed the 8-bit (unsigned) offset permitted. This fixes that as well,
by forcing it to always use evlddx/evstddx when accessing globals.
Part of the patch contributed by Kei Thomsen.