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[X86] Add DAG combine to replace vXi64 vzext_movl+scalar_to_vector with vYi32…

Authored by craig.topper on Mar 7 2020, 4:14 PM.

Description

[X86] Add DAG combine to replace vXi64 vzext_movl+scalar_to_vector with vYi32 vzext_movl+scalar_to_vector if the upper 32 bits of the scalar are zero.

We can just use a 32-bit copy and zero in the SSE domain when we
zero the upper bits.

Remove an isel pattern that becomes dead with this.

Details