Split record_header_indices loop to work around rustc/LLVM bug

rustc issue: rust-lang/rust#55105

Steps to reproduce:

```
rustup target add armv7-linux-androideabi
RUSTFLAGS="-Ctarget-feature=+neon" cargo build --target armv7-linux-androideabi --release
```

Output without this change:

```
   Compiling hyper v0.12.11 (/home/simon/projects/servo-deps/hyper)
LLVM ERROR: ran out of registers during register allocation
error: Could not compile `hyper`.
```

Author: SimonSapin

src/proto/h1/role.rs

@@ -924,6 +924,8 @@ fn record_header_indices(bytes: &[u8], headers: &[httparse::Header], indices: &m
         let name_start = header.name.as_ptr() as usize - bytes_ptr;
         let name_end = name_start + header.name.len();
         indices.name = (name_start, name_end);
+    }
+    for (header, indices) in headers.iter().zip(indices.iter_mut()) {
         let value_start = header.value.as_ptr() as usize - bytes_ptr;
         let value_end = value_start + header.value.len();
         indices.value = (value_start, value_end);