[llvm-mca] Default the output asm dialect used by the instruction printer to the input asm dialect.

The instruction printer used by llvm-mca to generate the performance report now
defaults the output assembly format to the format used for the input assembly
file.

On x86, the asm format can be either AT&T or Intel, depending on the
presence/absence of directive `.intel_syntax`.

Users can still specify a different assembly dialect with the command line flag
-output-asm-variant=<uint>.

llvm-svn: 330733