Fix for PR23310: llvm-dis crashes when trying to upgrade an intrinsic.

When trying to upgrade @llvm.x86.sse2.psrl.dq while parsing a module,
BitcodeReader adds the function to its worklist twice, resulting in a
crash when accessing it the second time.

This patch replaces the worklist vector by a map.

Patch by Philip Pfaffe.

llvm-svn: 241281