A load instruction will implicitely clear the top 32 bits when writing to a W
register. This patch avoids generating a `mov` instruction to zero-extend the
result in this case.

For example, this occurs in the generated code for dispatching to the next
bytecode in the interpreter:

  kind = BYTECODE_HANDLER
  name = LdaZero
  compiler = turbofan
  Instructions (size = 36)
  0x32e64c60     0  add x19, x19, #0x1 (1)
  0x32e64c64     4  ldrb w0, [x20, x19]
  0x32e64c68     8  mov w0, w0
                    ^^^^^^^^^^
  0x32e64c6c    12  lsl x0, x0, #3
  0x32e64c70    16  ldr x1, [x21, x0]
  0x32e64c74    20  movz x0, #0x0
  0x32e64c78    24  br x1

BUG=

Review-Url: https://codereview.chromium.org/1950013003
Cr-Commit-Position: refs/heads/master@{#36038}