LegalizeTypes support for legalizing the mask

operand of a VECTOR_SHUFFLE.  The mask is a
vector of constant integers.  The code in
LegalizeDAG doesn't bother to legalize the
mask, since it's basically just storage for
a bunch of constants, however LegalizeTypes
is more picky.  The problem is that there may
not exist any legal vector-of-integers type
with a legal element type, so it is impossible
to create a legal mask!  Unless of course you
cheat by creating a BUILD_VECTOR where the
operands have a different type to the element
type of the vector being built...  This is
pretty ugly but works - all relevant tests in
the testsuite pass, and produce the same
assembler with and without LegalizeTypes.

llvm-svn: 47670