Remove a redundant null-check in DIExpression::prepend, NFC

Code outside of an `if (Expr)` block dereferenced `Expr`, so the null
check was redundant.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@336465 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed