[MergeFunctions] Don't blindly RAUW a GlobalValue with a ConstantExpr.

MergeFunctions uses (through FunctionComparator) a map of GlobalValues
to identifiers because it needs to compare functions and globals
do not have an inherent total order. Thus, FunctionComparator
(through GlobalNumberState) has a ValueMap<GlobalValue *>.

r315852 added a RAUW on globals that may have been previously
encountered by the FunctionComparator, which would replace
a GlobalValue * key with a ConstantExpr *, which is illegal.

This commit adjusts that code path to remove the function being
replaced from the ValueMap as well.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316145 91177308-0d34-0410-b5e6-96231b3b80d8