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[x86] allow peeking through an extract_subvector to find a splatted operand

Authored by spatel on Feb 20 2020, 10:41 AM.

Description

[x86] allow peeking through an extract_subvector to find a splatted operand

The motivating case is seen in "splat4_v8f32_load_store" and based on code in PR42024:
https://bugs.llvm.org/show_bug.cgi?id=42024
(I haven't stepped through the v8i32 sibling test yet to see why that diverged.)

There are other potential improvements visible like allowing scalarization or vector
narrowing.

Differential Revision: https://reviews.llvm.org/D74909