[llvm-mca] Move the logic that updates the register files from InstrBuilder to Dispat...

[llvm-mca] Move the logic that updates the register files from InstrBuilder to DispatchUnit. NFCI

Before this patch, the register file was always updated at instruction creation
time. That means, new read-after-write dependencies, and new temporary registers
were allocated at instruction creation time.

This patch refactors the code in InstrBuilder, and move all the logic that
updates the register file into the dispatch unit. We only want to update the
register file when instructions are effectively dispatched (not before).

This refactoring also helps removing a bad dependency between the InstrBuilder
and the DispatchUnit.

No functional change intended.

llvm-svn: 327514