[X86] Add a DAG combine to combine INSERTPS and VBROADCAST of a scalar load....

[X86] Add a DAG combine to combine INSERTPS and VBROADCAST of a scalar load. Remove corresponding isel patterns.

We had an isel pattern to perform this, but its better to
do it in DAG combine as a simplification. This also fixes the lack
of patterns for AVX512 targets.

llvm-svn: 370294