Artem Pyanykh authored Apr 04, 2019 and Marge Bot committed Apr 09, 2019

Fixes #16052

When the offset in `setByteArray#` is statically known, we can provide
better alignment guarantees then just 1 byte.

Also, memset can now do 64-bit wide sets.

The current memset intrinsic is not optimal however and can be
improved for the case when we know that we deal with

(baseAddress at known alignment) + offset

For instance, on 64-bit

`setByteArray# s 1# 23# 0#`

given that bytearray is 8 bytes aligned could be unrolled into
`movb, movw, movl, movq, movq`; but currently it is
`movb x23` since alignment of 1 is all we can embed into MO_Memset op.