[X86] Remove a fptosi from the test_mm512_mask_reduce_max_pd fast-isel test.

The clang test inadvertently turned a floating point value into a double by having the wrong return type on the test function relative to the intrinsic it was testing.

This resulted in an extra fptosi instruction that propagated into this test when I copied the clang output.

llvm-svn: 335094