Re-apply r286006: Fix 24560: assembler does not share constant pool for same constants

Re-apply r286006: Fix 24560:  assembler does not share constant pool for same constants

Re-applying now that the open bug on this commit, PR32825, is known to be fixed.

Original commit message:
    Summary: This patch returns the same label if the CP entry with the same value has been created.

    Reviewers: eli.friedman, rengolin, jmolloy

    Subscribers: majnemer, jmolloy, llvm-commits

    Differential Revision: https://reviews.llvm.org/D25804

llvm-svn: 303539