This patch adds support for the `Operand2_R_LSL_I` addressing mode to
loads and stores. This allows merging a shift instruction into a
MemoryOperand. Since the shift immediate is restricted to the log2 of
the operation width, the opportunities to hit this are slim. However,
Ignition's bytecode handlers hit this case all the time:

kind = BYTECODE_HANDLER
name = Star
compiler = turbofan
Instructions (size = 44)
0x23e67280     0  add x1, x19, #0x1 (1)
0x23e67284     4  ldrsb x1, [x20, x1]
0x23e67288     8  sxtw x1, w1
0x23e6728c    12  mov x2, fp
0x23e67290    16  str x0, [x2, x1, lsl #3]
                  ^^^^^^^^^^^^^^^^^^^^^
0x23e67294    20  add x19, x19, #0x2 (2)
0x23e67298    24  ldrb w1, [x20, x19]
0x23e6729c    28  ldr x1, [x21, x1, lsl #3]
                  ^^^^^^^^^^^^^^^^^^^^^
0x23e672a0    32  br x1

Additionally, I noticed the optimisation occurs once in both the
`StringPrototypeCharAt` and `StringPrototypeCharCodeAt` turbofan stubs.

BUG=

Review-Url: https://codereview.chromium.org/1972103002
Cr-Commit-Position: refs/heads/master@{#36227}