Fix 24560: assembler does not share constant pool for same constants

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: 286006