Remove redundant casts. NFC

So I wrote a clang-tidy check to lint out redundant `isa`, `cast`, and
`dyn_cast`s for fun. This is a portion of what it found for clang; I
plan to do similar cleanups in LLVM and other subprojects when I find
time.

Because of the volume of changes, I explicitly avoided making any change
that wasn't highly local and obviously correct to me (e.g. we still have
a number of foo(cast<Bar>(baz)) that I didn't touch, since overloading
is a thing and the cast<Bar> did actually change the type -- just up the
class hierarchy).

I also tried to leave the types we were cast<>ing to somewhere nearby,
in cases where it wasn't locally obvious what we were dealing with
before.

llvm-svn: 326416
diff --git a/clang/lib/CodeGen/CGRecordLayoutBuilder.cpp b/clang/lib/CodeGen/CGRecordLayoutBuilder.cpp
index d794b12..7296da7 100644
--- a/clang/lib/CodeGen/CGRecordLayoutBuilder.cpp
+++ b/clang/lib/CodeGen/CGRecordLayoutBuilder.cpp
@@ -789,8 +789,7 @@
   }
                                      
   // Verify that the LLVM and AST field offsets agree.
-  llvm::StructType *ST =
-    dyn_cast<llvm::StructType>(RL->getLLVMType());
+  llvm::StructType *ST = RL->getLLVMType();
   const llvm::StructLayout *SL = getDataLayout().getStructLayout(ST);
 
   const ASTRecordLayout &AST_RL = getContext().getASTRecordLayout(D);