[llvm-mca] Remove field HasReadAdvanceEntries from class ReadDescriptor.

This simplifies the logic that updates RAW dependencies in the DispatchStage.
There is no advantage in storing that flag in the ReadDescriptor; we should
simply rely on the call to `STI.getReadAdvanceCycles()` to obtain the
ReadAdvance cycles. If there are no read-advance entries, then method
`getReadAdvanceCycles()` quickly returns 0.

No functional change intended.

llvm-svn: 335977