[CodeGen] allow printing of zero latency in sched comments

[CodeGen] allow printing of zero latency in sched comments

I don't know how to expose this in a test. There are ARM / AArch64
sched classes that include zero latency instructions, but I'm not
seeing sched info printed for those targets. X86 will almost
certainly have these soon (see PR36671), but no model has
'let Latency = 0' currently.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@327518 91177308-0d34-0410-b5e6-96231b3b80d8