[Hexagon] Improve code generation for 32x32-bit multiplication

[Hexagon] Improve code generation for 32x32-bit multiplication

For multiplications of 64-bit values (giving 64-bit result), detect
cases where the arguments are sign-extended 32-bit values, on a per-
operand basis. This will allow few patterns to match a wider variety
of combinations in which extensions can occur.

llvm-svn: 304223