[DAGCombine] Preserve shuffles when one of the vector operands is constant
Summary:
Do *not* perform combines such as:
vector_shuffle<4,1,2,3>(build_vector(Ud, C0, C1 C2), scalar_to_vector(X))
->
build_vector(X, C0, C1, C2)
Keeping the shuffle allows lowering the constant build_vector to a materialized
constant vector (such as a vector-load from the constant-pool or some other idiom).
Reviewers: delena, igorb, spatel, mkuper, andreadb, RKSimon
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D25524
llvm-svn: 285063