Add an option to emit instruction encodings as asm comments
Add an option to emit instruction encodings as asm comments

This mirrors a facility that llvm-mc has to do the same thing (and uses the same
option name: -show-encoding)

This decorates the asm output as follows:

s_lshr_b32 s0, s1, 15           ; encoding: [0x01,0x8f,0x00,0x90]
s_add_i32 s1, s0, 3             ; encoding: [0x00,0x83,0x01,0x81]
v_cvt_f32_u32_e32 v0, s1        ; encoding: [0x01,0x0c,0x00,0x7e]

This is sometimes useful when you want to confirm that the instruction encoding
really is what you think it is

googlestadia/llpc,GPUOpen-Drivers/llpc,googlestadia/llpc,googlestadia/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,googlestadia/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,googlestadia/llpc