[Power9]Legalize and emit code for converting Unsigned HWord/Char to Quad-Precision

Legalize and emit code for converting unsigned HWord/Char to QP:

xscvsdqp
xscvudqp

Only covering patterns for unsigned forms cause we don't have part-word
sign-extending integer loads into VSX registers.

Differential Revision: https://reviews.llvm.org/D45494

llvm-svn: 330278