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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@138768 91177308-0d34-0410-b5e6-96231b3b80d8