[X86] Add calculation for elements in structures in getting uniform base for...

[X86] Add calculation for elements in structures in getting uniform base for the Gather/Scatter intrinsic.

Summary: Add calculation for elements in structures in getting uniform
base for the Gather/Scatter intrinsic.

Reviewers: craig.topper, c-rhodes, RKSimon

Subscribers: hiraditya, llvm-commits, annita.zhang, LuoYuanke

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71442