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[x86] add movddup specialization for build vector lowering (PR37502)

Description

[x86] add movddup specialization for build vector lowering (PR37502)

This is admittedly a narrow fix for the problem:
https://bugs.llvm.org/show_bug.cgi?id=37502
...but as the XOP restriction shows, it's a maze to get this right.
In the motivating example, note that we have movddup before SSE4.1 and
again with AVX2. That's because insertps isn't available pre-SSE41 and
vbroadcast is (more generally) available with AVX2 (and the splat is
reduced to movddup via isel pattern).

Differential Revision: https://reviews.llvm.org/D55898

Details

Committed
spatelDec 21 2018, 10:48 AM
Differential Revision
D55898: [x86] add movddup specialization for build vector lowering (PR37502)
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rL349936: Add slides and remove expired sponsorship
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