Add support for expanding trunc stores. Consider

storing an i170 on a 32 bit machine.  This is first
promoted to a trunc-i170 store of an i256.  On a
little-endian machine this expands to a store of
an i128 and a trunc-i42 store of an i128.  The
trunc-i42 store is further expanded to a trunc-i42
store of an i64, then to a store of an i32 and a
trunc-i10 store of an i32.  At this point the operand
type is legal (i32) and expansion stops (legalization
of the trunc-i10 needs to be handled in LegalizeDAG.cpp).
On big-endian machines the high bits are stored first,
and some bit-fiddling is needed in order to generate
aligned stores.

llvm-svn: 43499