[clang-tidy] Use auto for readability

This only changes iterator type specifiers and cast expressions.

See
https://clang.llvm.org/extra/clang-tidy/checks/modernize-use-auto.html
for more on this warning.

Bug: v8:10488
Change-Id: I1e6747aafb24fd2d9d946e054fb20fbd1d8e5b26
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2191921Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67814}

src/compiler/backend/instruction.h

@@ -1536,7 +1536,7 @@ class V8_EXPORT_PRIVATE InstructionSequence final
     return virtual_register;
   }
   Constant GetConstant(int virtual_register) const {
-    ConstantMap::const_iterator it = constants_.find(virtual_register);
+    auto it = constants_.find(virtual_register);
     DCHECK(it != constants_.end());
     DCHECK_EQ(virtual_register, it->first);
     return it->second;