A bit of tuning of the arm64 isel: do PUT(..) = 0x0:I64 in a single insn.

A bit of tuning of the arm64 isel: do PUT(..) = 0x0:I64 in a single insn.

When running Memcheck, most blocks will do one and often two of `PUT(..) =
0x0:I64`, as a result of the way the front end models arm64 condition codes.
The arm64 isel would generate `mov xN, #0 ; str xN, [xBaseblock, #imm]`,
which is pretty stupid.  This patch changes it to a single insn:
`str xzr, [xBaseblock, #imm]`.

This is a special-case for `PUT(..) = 0x0:I64`.  General-case integer stores
of 0x0:I64 are unchanged.

This gives a 1.9% reduction in generated code size when running
/usr/bin/date on Memcheck.