Don't attempt to create VZEXT_LOAD out of an extload. This an issue where the

code generator would do something like this:

f64 = load f32 <anyext>, f32mem
v2f64 = insertelt undef, %0, 0
v2f64 = insertelt %1, 0.0, 1

into 

v2f64 = vzext_load f32mem

which on x86 is movsd, when you really wanted a cvtss2sd/movsd pair.

llvm-svn: 51624