[WebAssembly] Disable register coalescing at -O1

This disables `RegisterCoalescer` pass at -O1, which currently runs for
all levels except for -O0, as a part of common optimization pipeline.

`RegisterCoalescer` pass degrades Wasm debug info quality by a
significant margin. When I use `LiveDebugValue` analysis, disabling this
increases the average PC ranges covered by 15% on Emscripten core
benchmarks (52% -> 66.8%). (Our code is currently not using
`LiveDebugValues` analysis at the moment, and the experiment was done on
a local setting that enabled it. I'm planning to upstream it soon.)

In Emscripten core benchmarks, disabling this at -O1 causes +4.5% in
code size and +1% in the number of locals. The number of globals stays
the same. I believe this tradeoff is acceptable given that -O1 is not
usually used in production builds and is often used for debugging when
the application size is very large.

The plan is to investigate and fix what's causing the degradation in
that pass, but for now disabling it seems like a low-hanging quick fix.

Reviewed By: dschuff

Differential Revision: https://reviews.llvm.org/D138455