Handle IntToPtr in isBytewiseValue

Summary:
This helps with more efficient use of memset for pattern initialization

From @pcc prototype for -ftrivial-auto-var-init=pattern optimizations

Binary size change on CTMark, (with -fuse-ld=lld -Wl,--icf=all, similar results with default linker options)
```
                   master           patch      diff
Os           8.238864e+05    8.238864e+05       0.0
O3           1.054797e+06    1.054797e+06       0.0
Os zero      8.292384e+05    8.292384e+05       0.0
O3 zero      1.062626e+06    1.062626e+06       0.0
Os pattern   8.579712e+05    8.338048e+05 -0.030299
O3 pattern   1.090502e+06    1.067574e+06 -0.020481
```

Zero vs Pattern on master
```
               zero       pattern      diff
Os     8.292384e+05  8.579712e+05  0.036578
O3     1.062626e+06  1.090502e+06  0.025124
```

Zero vs Pattern with the patch
```
               zero       pattern      diff
Os     8.292384e+05  8.338048e+05  0.003333
O3     1.062626e+06  1.067574e+06  0.003193
```

Reviewers: pcc, eugenis

Subscribers: hiraditya, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D63967

llvm-svn: 365858