[llvm-mca][RISCV] vsetivli and vsetvli act as instruments

Since the LMUL data that is needed to create an instrument is
avaliable statically from vsetivli and vsetvli instructions,
LMUL instruments can be automatically generated so that clients
of the tool do no need to manually insert instrument comments.

Instrument comments may be placed after a vset{i}vli instruction,
which will override instrument that was automatically inserted.
As a result, clients of llvm-mca instruments do not need to update
their existing instrument comments. However, if the instrument
has the same LMUL as the vset{i}vli, then it is reccomended to
remove the instrument comment as it becomes redundant.

Differential Revision: https://reviews.llvm.org/D154526