If you're generating a .s file for X86, you can specify the flavor of
assembly language on the command line like this:
llvm-gcc ... -mllvm -x86-asm-syntax=[att|intel]
Here's the relevant lines in the llc -help:
...
-x86-asm-syntax - Choose style of code to emit
from X86 backend:
=att - Emit AT&T-style assembly
=intel - Emit Intel-style assembly
Initially it was thought to emit assembler compatible with MASM.
However, MASM is too weak to assemble something needed, for example,
for more or less non-trivial C++ (and sometimes even C!) source.