[LegalizeVectorTypes] When widening the result of a bitcast from a scalar...

[LegalizeVectorTypes] When widening the result of a bitcast from a scalar type, use a scalar_to_vector to turn the scalar into a vector intead of a build vector full of mostly undefs.

This is more consistent with what we usually do and matches some code X86 custom emits in some cases that I think I can cleanup.

The MIPS test change just looks to be an instruction ordering change.

llvm-svn: 344422