Metadata: Optimize metadata queries (#70700)

Optimize metadata query code:
- Avoid `DenseMap::operator[]` in situations where it is known that the
key exists in the map. Instead use `DenseMap::at()`/
`DenseMap::find()->second`. This avoids code-bloat and bad inlining
decisions for the unused insertion/growing code in `operator[]`.
- Avoid a redundant `Value::hasMetadata()` check.
- Move the `KindID == LLVMContext::MD_dbg` case to
`Instruction::getMetadata` and check it first assuming that it can be
constant folded after inlining in many situations.

The motivation for this change is a regression triggered by
e3cf80c5 which could attributed to the
compiler inlining the insertion part of `DenseMap::operator[]` in more
cases while unbeknownst to a compiler (without PGO) that code is never
used in this context. This change improves performance and eliminates
difference before and after that change in my measurements.