PTX: Default to 32-bit indexing of pointers.

[maleadt]

This change sets the default index size for address calculations in PTX code to 32bit, in order to avoid extensions of 32-bit indices to 64-bit as part of instcombine when generating new GEP instructions (see JuliaLLVM/LLVM.jl#342 (comment)).

I'm not entirely sure this is correct, but it looks safe. It doesn't limit address calculations to 4GB, as you can still use 64-bit indices, but just defaults to 32-bit as these are generally more GPU friendly.
FWIW, the NVPTX back-end docs leave the index size undefined, and have it default to 64 bits.

x-ref JuliaGPU/CUDA.jl#1895

cc @vchuravy

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -1.77 ⚠️

Comparison is base (5de5a5c) 87.06% compared to head (a514365) 85.30%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #444      +/-   ##
==========================================
- Coverage   87.06%   85.30%   -1.77%     
==========================================
  Files          24       24              
  Lines        2961     2994      +33     
==========================================
- Hits         2578     2554      -24     
- Misses        383      440      +57     

Impacted Files	Coverage Δ
src/ptx.jl	93.80% <100.00%> (-0.03%)	⬇️

... and 9 files with indirect coverage changes

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[maleadt]

Let's try this out...