[SelectionDAG] Move HwMode expansion from tablegen to SelectionISel. (#174471)

The way HwMode is currently implemented, tablegen duplicates each
pattern that is dependent on hardware mode. The HwMode predicate is
added as a pattern predicate on the duplicated pattern.
    
RISC-V uses HwMode on the GPR register class which means almost every
isel pattern is affected by HwMode. This results in the isel table
being nearly twice the size it would be if we only had a single GPR
size.

This patch proposes to do the expansion at instruction selection time
instead. To accomplish this new opcodes like OPC_CheckTypeByHwMode
are added to the isel table. The unique combinations of types and HwMode
are converted to an index that is the payload for the new opcodes.
TableGen emits a new virtual function getValueTypeByHwMode that uses
this index and the current HwMode to look up the type.

This reduces the size of the isel table on RISC-V from ~2.38 million
bytes to ~1.38 million bytes.

I did not add an OPC_SwitchTypeByHwMode opcode yet. If the VT requires a
hardware mode, we emit an OPC_Scope+OPC_CheckTypeByHwMode instead. I
expect adding an OPC_SwitchTypeByHwMode could further reduce the table
size. I will investigate this as a follow up.
    
Many of the matcher classes in tablegen now use ValueTypeByHwMode
insteadof MVT. This may have an impact on the memory usage and runtime of
tablegen. We can mitigate some of this by splitting the matchers into MVT and
ValueTypeByHwMode versions. We can also explore alternate data
structures for ValueTypeByHwMode instead of a std::map. Maybe a sorted vector.

A similar change can be made to GlobalISel as a follow up.