[SLP] Limit GEP lists based on width of index computation.

D68667 introduced a tighter limit to the number of GEPs to simplify

together. The limit was based on the vector element size of the pointer,

but the pointers themselves are not actually put in vectors.

IIUC we try to vectorize the index computations here, so we should base

the limit on the vector element size of the computation of the index.

This restores the test regression on AArch64 and also restores the

vectorization for a important pattern in SPEC2006/464.h264ref on

AArch64 (@test_i16_extend). We get a large benefit from doing a single

load up front and then processing the index computations in vectors.

Note that we could probably even further improve the AArch64 codegen, if

we would do zexts to i32 instead of i64 for the sub operands and then do

a single vector sext on the result of the subtractions. AArch64 provides

dedicated vector instructions to do so. Sketch of proof in Alive:

https://alive2.llvm.org/ce/z/A4xYAB

Reviewers: craig.topper, RKSimon, xbolva00, ABataev, spatel

Reviewed By: ABataev, spatel

Differential Revision: https://reviews.llvm.org/D82418