Clean up the handling of inline asm.

Make inline asm a report_fatal_error instead of an assertion failure so
that it's a little friendlier, and add a test to make sure llc -march=js
rejects inline asm.

Also disable the PNaCl inline asm("":::"memory") lowering pass. If people
are using this, it's best that we diagnose it as it likely isn't portable.
There are usually better alternatives.