ARM assembler: fix undefined behaviour in fits_shifter

Bit-shifts have undefined behaviour if the shift amount is greater
or equal to the width of the type.

In this case the code would do imm32 >> 32 when rot == 0.

A newer version of Clang unrolled the loop, optimized the first
iteration away, causing the test suite to fail with:

  #
  # Fatal error in ../src/arm/assembler-arm.cc, line 1212
  # Check failed: !rn.is(ip).
  #

as well as crashing when running Chromium tests on Android (at least
we think this was the cause, see the bug).

BUG=463436, 444089
LOG=Y

Review URL: https://codereview.chromium.org/979633002

Cr-Commit-Position: refs/heads/master@{#26974}

src/arm/assembler-arm.cc

@@ -1011,7 +1011,8 @@ static bool fits_shifter(uint32_t imm32,
                          Instr* instr) {
   // imm32 must be unsigned.
   for (int rot = 0; rot < 16; rot++) {
-    uint32_t imm8 = (imm32 << 2*rot) | (imm32 >> (32 - 2*rot));
+    uint32_t imm8 =
+        rot == 0 ? imm32 : (imm32 << 2 * rot) | (imm32 >> (32 - 2 * rot));
     if ((imm8 <= 0xff)) {
       *rotate_imm = rot;
       *immed_8 = imm8;