Don't combine a shuffle of two BUILD_VECTORs with duplicate elements.
Targets can't handle this case well in general; we often transform
a shuffle of two cheap BUILD_VECTORs to element-by-element insertion,
which is very inefficient.
Fixes https://llvm.org/bugs/show_bug.cgi?id=31364 . Partially
fixes https://llvm.org/bugs/show_bug.cgi?id=31301.
Differential Revision: https://reviews.llvm.org/D27787
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@289874
91177308-0d34-0410-b5e6-
96231b3b80d8
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