[CostModel][X86] Add i64 mul cost for avx512 as 1cy

i64 mul cost is 1cy for all cpu that support avx512. Currently
all X86 cpu uses i64 mul cost in X64 cost table which is not
true for cpu that support avx512 (skx, icx).

Reviewed By: pengfei, RKSimon

Differential Revision: https://reviews.llvm.org/D115016