[X86] Use a signed mask in foldMaskedShiftToScaledMask to enable a shorter immediate encoding.

This function reorders AND and SHL to enable the SHL to fold into an LEA. The
upper bits of the AND will be shifted out by the SHL so it doesn't matter what
mask value we use for these bits. By using sign bits from the original mask in
these upper bits we might enable a shorter immediate encoding to be used.

llvm-svn: 357846