[SPIRV] Rename `selectSelectDefaultArgs` to `selectBoolToInt` (#184120)
The function is used to extend a `bool` (vector or scalar) into `1/-1`
for `true` and `0` for `false` (vector or scalar).
There is no obvious "default" argument for a select operation, so the
original name is confusing.
This patch:
* Renames this function to better signal its intention,
* makes the boolean argument explicit in the function (instead of
implicit through the first register operand of the instruction),
* rename `I` to `InsertAt`.