Avoid creating a Constant for each value in a ConstantDataSequential.

Summary: We create a ConstantDataSequential (ConstantDataArray or ConstantDataVector) to avoid creating a Constant for each element in an array of constants. But them in AsmPrinter, we do create a ConstantFP for each element in the ConstantDataSequential. This triggers excessive memory use when generating large global FP constants.

Reviewers: bogner, lhames, t.p.northover

Subscribers: jlebar, sanjoy, llvm-commits

Differential Revision: https://reviews.llvm.org/D44277

llvm-svn: 327161