[AMDGPU][HIP] Improve opt-level handling

Summary:
The HIP toolchain invokes `llc` without an explicit opt-level, meaning
it always uses the default (-O2). This makes it impossible to use -O1,
for example. The HIP toolchain also coerces -Os/-Oz to -O2 even when
invoking opt, and it coerces -Og to -O2 rather than -O1.

Forward the opt-level to `llc` as well as `opt`, and only coerce levels
where it is required.

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70987