Change IndexedModeAction representation.

This changes the IndexedModeAction representation to remove the
limitation on the number of value types in MVT.  This limitation
prevents us from specifying AVX types.

Prior to this change IndexedModActions was represented as follows...

uint64_t IndexedModeActions[2][ISD::LAST_INDEXED_MODE];

the first dimension was used to represent loads, then stores.  This
imposed a limitation of 32 on the number of value types that could be
handled with this method.  The value type was used to shift the two bits
into and out of the approprate bits in the uint64_t.

With this change the array is now represented as ...

uint8_t IndexedModeActions[MVT::LAST_VALUETYPE][2][ISD::LAST_INDEXED_MODE];

Takes more space but removes the limitation on MVT::LAST_VALUETYPE.  The
first dimension is now the value_type for the reference.  The second
dimension is the load [0] vs. store[1].  The third dimension represents
the various modes for load store.  Accesses are now direct, no shifting
or masking.

There are other limitations that need to be removed, so that
MVT::LAST_VALUETYPE can be greater than 32.  This is merely the first
step towards that goal.

llvm-svn: 73104