Recognize ValueType operands in source patterns for fast-isel.

Currently the fast-isel table generator recognizes registers, register
classes, and immediates for source pattern operands.  ValueType
operands are not recognized.  This is not a problem for existing
targets with fast-isel support, but will not work for targets like
PowerPC and SPARC that use types in source patterns.

The proposed patch allows ValueType operands and treats them in the
same manner as register classes.  There is no convenient way to map
from a ValueType to a register class, but there's no need to do so.
The table generator already requires that all types in the source
pattern be identical, and we know the register class of the output
operand already.  So we just assign that register class to any
ValueType operands we encounter.

No functional effect on existing targets.  Testing deferred until the
PowerPC target implements fast-isel.