[libclc] Avoid out-of-range float-to-int. (#145698)

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.

GitOrigin-RevId: 46ee7f1908f467d8feb5617e097c88be1401296c