Explicitly zero out parts of a vector which are required to be zero by the...

Explicitly zero out parts of a vector which are required to be zero by the algorithm in LowerUINT_TO_FP_i32.  This only has a substantial effect on the generated code when the input is extracted from a vector register; other ways of loading an i32 do the appropriate zeroing implicitly.  Fixes PR10802.

llvm-svn: 138768