Merging ISD::LOAD and ISD::LOADX. Added LoadSDNode to represent load nodes.

Chain and address ptr remains as operands. SrcValue, extending mode, extending
VT (or rather loaded VT before extension) are now instance variables of
LoadSDNode.

Introduce load / store addressing modes to represent pre- and post-indexed
load and store. Also added an additional operand offset that is only used in
post-indexed mode (i.e. base ptr += offset after load/store).

Added alignment info (not yet used) and isVolatile fields.

llvm-svn: 30843