Prefer using eax to isolate the sign bit. This saves a byte when the
destination ends up as r8-15, because those require a REX prefix.
Before:
41 8B C5 mov eax,r13d
41 C1 ED 1F shr r13d,1Fh
44 03 E8 add r13d,eax
41 D1 FD sar r13d,1
After:
41 8B C5 mov eax,r13d
C1 E8 1F shr eax,1Fh
44 03 E8 add r13d,eax
41 D1 FD sar r13d,1