[DXIL] Add `log`, `log2`, and `log10` intrinsic Lowering

[farzonl]

Needs to cover the float and half case for all three: https://godbolt.org/z/64fzEq3nd

This is a perfect candidate for instruction expansion:

• log2
  • This is the simplest case just call unary dxilop 23
• log
  • Call log2 DXIL op (ie 23)
  • Create a ln2 const value
    • M_LN2_F 0.693147180559945309417232121458176568f
    • M_LN2_H 0.693147180559945309417232121458176568h
  • Floating point multiply the value of M_LN2 with the return value of log2
  • Reference implementation
• log10
  • Define M_LN10
    • M_LN10_F 2.30258509299404568401799145468436421f
    • M_LN10_H 2.30258509299404568401799145468436421h
  • Call log2 DXIL op (ie 23)
  • Create a log2_10Const = M_LN2 / M_LN10 const value
• Floating point multiply the value of log2_10Const with the return value of log2
• Reference implementation