ARM: lower fpowi appropriately for Windows ARM

This handles the last case of the builtin function calls that we would
generate code which differed from Microsoft's ABI.  Rather than
generating a call to `__pow{d,s}i2` we now promote the parameter to a
float or double and invoke `powf` or `pow` instead.

Addresses PR30825!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@286082 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed