X86: Use a smaller 'mov' instruction for stack probe calls

Prologue emission, in some cases, requires calls to a stack probe helper
function.  The amount of stack to probe is passed as a register
argument in the Win64 ABI but the instruction sequence used is
pessimistic: it assumes that the number of bytes to probe is greater
than 4 GB.

Instead, select a more appropriate opcode depending on the number of
bytes we are going to probe.

llvm-svn: 230270