[libclc] Avoid out-of-range float-to-int.

[hvdijk]

For a kernel such as

kernel void foo(__global double3 *z) {
  double3 x = {0.6631661088,0.6612268107,0.1513627528};
  int3 y = {-1980459213,-660855407,615708204};
  *z = pown(x, y);
}

we were not storing anything to z, because the implementation of pown relied on an floating-point-to-integer conversion where the floating-point value was outside of the integer's range. Although in LLVM IR we permit that operation so long as we end up ignoring its result -- that is the general rule for poison -- one thing we are not permitted to do is have conditional branches that depend on it, and through the call to __clc_ldexp, we did have that.

To fix this, rather than changing expv at the end to INFINITY/0, we can change v at the start to values that we know will produce INFINITY/0 without performing such out-of-range conversions.

Tested with

clang --target=nvptx64 -S -O3 -o - test.cl \
  -Xclang -mlink-builtin-bitcode \
  -Xclang runtimes/runtimes-bins/libclc/nvptx64--.bc

A grep showed that this exact same code existed in three more places, so I changed it there too, though I did not do a broader search for other similar code that potentially has the same problem.

[frasercrmck]

LGTM